Cavity Ring-Down Spectroscopy (CRDS) is an increasingly widely used technique for detecting and monitoring analytes, especially when the target analyte is present in very low concentration. Techniques are available which enable the use of CRDS with gaseous, liquid or solid samples. Various aspects of CRDS are described in numerous U.S. Patents such as U.S. Pat. Nos. 5,815,277, 5,903,358, 5,912,740, 6,084,682, 6,094,267, 6,233,052, 6,377,350, 6,452,680, 6,466,322 and 6,532,071. Cavity Ringdown Spectroscopy by K. W Busch and M. A Busch, ACS Symposium Series No 720, 1999 ISBN 0-8412-3600-3, gives a comprehensive, generally up to date overview of many aspects of CRDS technology.
In essence, CRDS involves measuring the decay time of a photon filled, high finesse resonant optical cavity (the ring-down cavity). The cavity is formed by from two to usually three or four ultra-high reflectivity dielectric mirrors, which comprise the optical resonator. Monochromatic light from a laser is injected into the cavity which encloses the analyte sample. The decay time is determined by:                i) the round trip path length of the optical beam within the cavity;        ii) losses inherent in the cavity itself (primarily diffraction losses and losses through the mirrors); and        iii) most importantly, losses due to the frequency dependent losses due to absorption by the target analyte.        
Since losses i) and ii) are independent of the analyte, the analyte spectrum is determined by the frequency dependent decay time of the resonant cavity with the target analyte present.
A major advantage of CRDS relative to conventional absorption spectroscopy is that it does not depend on a power-ratio measurement but rather provides an absolute measurement (i.e, decay time).
As above-indicated, cavity ring-down spectroscopy involves measuring the absorption of radiation by a sample via the effects of this absorption on the decay rate (the “ring-down time constant”) of an optical cavity. The absorption is measured as a function of the wavelength of light resonating in the cavity to obtain the desired spectrum and/or concentration of a target analyte.
The optical cavity is initially filled with radiation from a laser, and ring-downs are initiated by interrupting this incoming radiation. It is important for the purposes of high-resolution spectroscopy that the wavelength of the laser be precisely known at the time each ring-down occurs. In the present invention the cavity length is adjusted so that (a mode of) the cavity is in resonance with the laser radiation at the time of ring-down. Due to the resonance condition, the intra-cavity intensity builds up rapidly while the laser is on. The accuracy to which the ring-down time constant can be measured improves with increasing intra-cavity optical intensity, so it is desirable to make this quantity (the “cavity filling”) as large as possible.
Several factors limit how much cavity filling can be achieved in practice. Due to the very high finesse (or very narrow line width) of the cavity, small fluctuations in the laser wavelength (or the cavity length) can cause the incident light to go into and out of resonance with the cavity. When this happens, the intra-cavity intensity may decrease or fluctuate irregularly while the laser remains turned on. In addition, filling uniformity also affects the repetition rate and hence measurement speed. It is therefore doubly important for the laser to have minimal frequency jitter. For a semiconductor laser, the wavelength is a very sensitive function of both the pump current to, and the temperature of, the laser, making the control of these quantities very important. In particular, achieving good cavity filling at each of a collection of wavelengths (as required for a spectral scan) requires a very low-noise current source and the ability to control the laser pump current over a moderately wide bandwidth (of the order of the inverse cavity lifetime) to maintain the laser output at the desired wavelength set point while the cavity fills up.
A conventional two-mirror, continuous wave (CW) CRDS instrument (200) is shown in FIG. 1.
As shown in FIG. 1, light is generated from a narrow band, tunable, continuous wave diode laser 202. Laser 202 is temperature tuned by a temperature controller (not shown) to emit its radiation at a wavelength approximately equal to a desired spectral line of the analyte. An acousto-optic modulator (AOM) 204 is positioned in front of the radiation emitted from laser 202. AOM 204 provides a means for providing light 206 from laser 202 along the optical axis 219 of resonant cavity 218. Light 206 exits AOM 204 and is directed by mirrors 208 and 210 to cavity mirror 220 as light 206a which travels along optical axis 219 and exponentially decays between cavity mirrors 220 and 222 when light 206 is extinguished or deflected from the cavity axis. The measure of this decay is indicative of the presence or lack thereof of a trace species. Detector 212 is coupled between the output of optical cavity 218 and controller 214. Controller 214 is coupled to laser 202, processor 216, and AOM 204. Processor 216 processes signals from optical detector 212 in order to determine the level of trace species in optical resonator 218.
In AOM 204, a pressure transducer (not shown) creates a sound wave that modulates the index of refraction in an active nonlinear crystal (not shown), through a photoelastic effect. The sound wave produces a Bragg diffraction grating that disperses incoming light into multiple orders, predominantly zero order and first order. Different orders have different light beam energy and follow different beam directions. In CW-CRDS, typically, a first order light beam 206 is aligned along with optical axis 219 of cavity 218 incident on the cavity in-coupling mirror 220, and a zero order beam 224 is idled with a different optical path (higher order beams are very weak and thus not addressed). Thus, AOM 204 controls the direction of beams 206 and 224.
When AOM 204 is on, most light power (typically, up to 80%, depending on size of the beam, crystals used in AOM 204, alignment, etc.) goes to the first order along optical axis 219 as light 206. The remaining beam power goes to the zero order (light 224), or higher orders. The first order beam 206 is used for the input coupling light source; the zero order beam 224 can be used for diagnostic components. Once sufficient light energy is built up within the cavity. AOM 204 is turned off. This results in all the beam power going to the zero order as light 224, and no light 206 is coupled into resonant cavity 218. The light energy inside the cavity then follows an exponential decay (i.e., “rings down”).
In order to “turn off” the laser light to optical cavity 218, and thus allow for energy within optical cavity 218 to ring down, AOM 204, under the control of controller 214, redirects (deflects) light from laser 204 along path 224 and thus away from optical path 206 into optical resonator 218.
Some art workers have endeavored to provide an alternative to using an AOM to turn off the transmission of photons into the optical cavity. For example, the system described in WIPO applications 03/098173 (US 2003/0210398) is reported to function as follows:                i) a controller deactivates (shuts off) the laser light source when the light emitted from the cavity reaches a predetermined threshold. The laser is turned off by shunting the laser current away from the laser;        ii) the laser remains shut off for a fixed period significantly exceeding the ring-down time and the cavity rings-down during the initial portion of the fixed shut-off period and the concentration of the target analyte is thereby measured;        iii) the light source is turned back on at the end of this first shut-off period to thereby initiate a second fixed period during which the restarted laser “stabilizes”. By setting the laser temperature to an appropriate value, by the end of this period the laser emission frequency should be stabilized at a value which is approximately correct for a given target analyte. The current to the laser is then modulated to more finely vary the laser emission frequency until it coincides with a cavity resonance mode at some point during the modulation resulting in energy build-up within the fixed length cavity. While this system may sometimes have advantages over a system using an AOM to turn off the light into the optical cavity, it is not capable of achieving the degree of precision achievable with an optimized CRDS instrument in accordance with the present invention.        
Since the laser emission wavelength depends both on temperature and pump current, adjusting the laser temperature to a preset value so as to provide a nominal (in reality only approximately correct) wavelength, and subsequently modulating the current, can cause the actual laser wavelength to differ significantly from the desired value. It is not possible to do high-resolution spectroscopy with such a configuration. Equally significant, in this method no mechanism to compensate for the effects of laser aging. It is well known in the laser art that over time the temperature and current required to achieve a particular emission wavelength will change. The above design does not provide a feedback mechanism to detect or compensate for the effects of aging so that over time the instrument will tend to drift away from the analyte absorption feature which it is trying to detect.